Radar monopulse antennas with converting polarization

ABSTRACT

A monopulse antenna comprising an assymetrical parabolic reflector illuminated by a monopulse horn. A circular polarizing array cover adapted for selective emplacement over the horn aperture is provided. Changing from linear to circular polarization by placing the polarizer, by itself, over the horn produces an angular shift in antenna boresite (axis). A dielectric lens is provided for insertion over the polarizer to introduce a linear corrective phase shift, thereby eliminating the said axis shift.

United States Patent Beguin 5] Oct. 24, 1972 [54] RADAR MONOPULSEANTENNAS WITH CONVERTING POLARIZATION [72] Inventor: Daniel EdmondBeguln, Saint-Prix,

France [73] Assignee: International Standard Corporation,

New York, NY.

[22] Filed: June 23, 1971 [21] Appl. No.: 155,940

[30] Foreign Application Priority Data June 25, 1970 France ..7023592[52] US. Cl. ..343/755, 343/756, 343/786 [51] Int. Cl. ..l-l0lq 19/00[58] Field of Search ..343/754, 755, 756, 786, 840, 343/909 [56]References Cited UNITED STATES PATENTS 2,934,762 4/1960 Smedes ..343/755Primary E xaminen-Eli Lieberman Attorney-C. Cornell Remsen, Jr. et al.

[57] ABSTRACT A monopulse antenna comprising an assymetrical parabolicreflector illuminated by a monopulse horn. A circular polarizing arraycover adapted for selective emplacement over the horn aperture isprovided. Changing from linear to circular polarization by placing thepolarizer, by itself, over the horn produces an angular shift in antennaboresite (axis). A dielectric lens is provided for insertion over thepolarizer to introduce a linear corrective phase shift, therebyeliminating the said axis shift.

8 Claims, 6 Drawing Figures PATENTEDnm 24 m2 SHEET 1 OF 2 Fig-2 Inventor Dame/ E. Bey/1m A grill g(negcltive) RADAR MONOPULSE ANTENNASWITH CONVERTING POLARIZATION CROSS-REFERENCE TO RELATED APPLICATIONS TheU.S. Pat. application for this case is filed with claim for prioritypursuant to 35 U.S.C. 119, based on an application for the sameinvention filed in France on June 25, 1970, Ser. No. 7,023,592.

BACKGROUND OF THE INVENTION 1. Field of the Invention The presentinvention relates generally to radar antenna systems, and moreparticularly, to monopulse (simultaneous lobing) antenna systems withlinear/circular polarization selection.

2. Description of the Prior Art At the outset, it will be recalled thata simultaneous lobing or monopulse radar makes it possible to measuremagnitude and angle between the antenna axis (boresite) and a target. Inan amplitude comparison monopulse radar, two primary sources areprovided for each angular coordinate, these are simultaneouslyilluminating the same reflector. The two beams thus generated arephysically located so that their radiation patterns are overlapping. Theamplitudes of the received signals on both primary sources arerespectively added in-phase and in phase opposition so that the sumsignal S and difference signal D obtained are in phase for an error ofone sense and out of phase for an error of the other sign. Such signalsare respectively amplified in a sum reception channel and in adifference reception channel, the output signals of which are applied toa demodulator circuit. Such a demodulator circuit performs the operationD/S and delivers a signal whose amplitude is substantially proportional,for small angles of deviation, to the magnitude of the deviation (angleerror magnitude) and whose polarity depends on the error sign (sensewith respect to th said axis).

The above-mentioned two primary sources are conveniently obtainedthrough use of a rectangular crosssection horn comprising a medianpartition. The two horn outputs thus available are connected to a hybridjunction (magic-T, for example) which delivers signals respectivelyequal to the sum and to the difference of energies received by the twohorn sections.

In radar techniques, it is known that for suppressing rain echoes,transmitting waves having circular polarization affords some significantdiscrimination against rain return. Such circular polarization wavesare, for example, obtained through the use of a parallel metal platedevice, called a polarizer, which, in the case of an antenna for amonopulse radar, is typically located in front of (over the opening of)the horn. The polarizer is located with its plane parallel to the planeof the horn opening and is oriented in such a manner that its platesmake a 45 angle with respect to the linear polarization direction. Theradiated field component which is perpendicular to the plates isunaffected by the polarizer, however, the component which is parallel tothe said plates has its phase advanced by 90 and, consequently, the waveradiated from the polarizer is circularly polarized.

Since reflection coefficients of a statistical average rain drop areequal for the two circular polarization wave components, the wavereflected by a rain drop illuminated thereby is circularly polarized.When this reflected wave passes through the polarizer, the componentwhich is parallel to the plates again experiences a phase advancement by90, with the result that, after having been combined with the othercomponent, the resulting linear polarization is perpendicular to theinitially transmitted wave polarization. As the horn constitutes afilter for waves having a polarization direction perpendicular to thatfor which it has been designed, waves reflected by rain drops results ina significant signal rejection at the polarizer. Rain echoes are therebysuppressed.

Other reflecting objects may present similar reflection coefficientsthereby producing echo signals having rain-like polarization. Suchsignals would be similarly suppressed. Therefore, it is not desirable tocontinuously radiate in the circular polarization mode when no rainproblem is extant. Accordingly, the polarizer is usually mounted on amechanical device which makes it possible to emplace it on or remove itfrom the horn opening at will. I

It will be noted that an operative polarizer may be located either atthe horn output as hereabove, on the reflector itself, or behind thehorn in the path of the waves reflected by the reflector. The mostpractical of these possibilities is generally considered to be the hornoperative grating in that it is relatively compact compared to the otherforms.

The'parabolic reflector consists of a parabolic surface of revolutionthe focus of which coincides with the horn location. Due to the hornlocation, the horn acts as a mask producing aperture blockage. In theprior art, one of the expedients for eliminating aperture blockage inthis type of antenna system involves construction of the reflector as apartial surface of revolution with an offset feed oriented to illuminatethis surface. Such-a horn and reflector arrangement is depicted at FIG.7.10 of the textbook, Introduction to Radar Systems, by Merrill I.Skolnik, a McGraw-Hill book (1962). Such an arrangement with removablepolarizer located between the horn and the reflector, when used for amonopulse radar, is subject to a shift of the antenna axiswhen changingfrom linear polarization to circular polarization. An angular error thusarises.

The manner in which the present invention eliminates this problem ishereinafter described.

SUMMARY OF THE INVENTION ing said reflector and providing a plane wave,a

removable polarizer located between said primary source and saidreflector, and means inserted between said polarizer and said reflectorfor introducing a phase shift varying linearly over the entire beamwidth from substantially zero to a predetermined value.

The above-mentioned and other features and objects of this inventionwill become apparent by reference to the following description inconjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 shows severalvector-resolutions of the electric field at the reflector surface, forexplanation of the background problem solved by the invention,

FIG. 2 shows two patterns of the electric field at the horn opening,

FIGS. 3(a) 3(b) and 3(0) illustrate radiation patterns which furtherdescribe the relationships producing antenna axis shift in circularpolarization, and

FIG. 4 shows a cross-section of an antenna according to the presentinvention.

DESCRIPTION OF THE PREFERRED EMBODIMENT Before beginning the descriptionof the present invention, an explanation in conjunction with the FIGS.1, 2, 3(a), 3(b) and 3(c) will be given to provide an understanding ofthe reason why passing from linear polarization to circular polarizationresults in the antenna axis drift which is the problem to which thepresent invention is addressed.

in FIG. 1, the lines of force F of the electric field at the surface ofa parabolic partial surface illuminated by a vertical polarizationprimary source are shown. Asingle line of force F, corresponding tohorizontal polarization excitation is also drawn. The vectors of theelectric fields E and E as well as the vectors resulting from theirresolution among the vertical axis YOY and the horizontal axis X'OX areshown in each of two cases, viz.; one (indicia p) when the primarysource radiation pattern has an even symmetry, the other (indicia i)when the primary source radiation pattern has an odd symmetry. Thesesymmetries correspond to sum and difference patterns, respectively, inmonopulse operation. FIG. 2 illustrates these two electric fielddistributions in the horn aperture, the said even symmetry pattern beingcurve S and the odd symmetry pattern being curve D.

If the entire surface of revolution is illuminated, the vectorialresolution shows that there exists a spurious component, which ishorizontal when polarization is vertical, and vertical when polarizationis horizontal. The effects of that spurious component are compensated inthe far electric field; however, such a compensation does not take placewhen only the upper half of the complete parabolic portion is employed(unhatched part, FIG. 1). That case corresponds to the aforementionedoffset feed situation. Thus, in the case of the vertical polarizationeven symmetry primary radiation pattern, the horizontal components inthe quadrants X'OY and XOY are mutually 180 out of phase and correspondto the odd symmetry radiation pattern as illustrated by the curve Sh ofFIG. 3(a). The pattern generated by the main vertical component EpV isillustrated by the curve SV [FIG. 3(a)] whose amplitude is on the orderof twenty decibels greater than that of the curve Sh.

If it is assumed that the phase of the pattern SV is zero, the phase ofthe spurious pattern Sh will be -n'/2 at the left side (negativebearings) from the axis since it is necessary to turn by an angle 1r/2in the direction opposite to the trigonometric direction (sign from themain vertical component EpV to the spurious horizontal component Eph.Similar considerationsfor the spurious pattern Sh located in thepositive bearing side shows that its phase is IT/2.

In the case of a vertical polarization, odd symmetry primary radiationpattern illuminating the upper half of the parabolic portion, thevectorial resolution shows that the main vertical component EiV has anodd pattern which corresponds to the radiation pattern DV of FIG. 3(a)while the spurious horizontal component Eih has an even symmetry whichcorresponds to the radiation pattern Dh of FIG. 3(a). The phase of thepattern Dh may be found according to the same previously mentionedconsiderations, i.e., a phase equal to zero is obtained.

As, on the one hand, the patterns SV and DV and, on the other hand, Shand Dh have crossed polarization, these latter ones do not disturb themain patterns SV and DV.

Similar consideration, as hereabove, given to the case of a horizontalpolarization primary source, would show that the patterns resulting fromthe spurious vertical component [curves Svd and Dvd of FIG. 3(b)] do notdisturb the horizontal polarization main patterns [curves SI-Id and DHdof the FIG. 3(b)]. In FIG. 1, the components to be considered arereferenced E'p, E'pH, E'pv for the even symmetry pattern, and E'iI-I,Eiv for the odd symmetry pattern.

The conditions described described for linear polarization no longerhold true if the primary source radiates circular polarization waves,the representative vector of which may be reduced to vertical andhorizontal orthogonal components. As a matter of fact, the circularpolarization case amounts to coexistence of the radiation patterns ofthe FIGS. 3(a) and 3(b), the amplitudes in the same'polarization planein the same channel being added or subtracted depending on theirrelative phases.

By way of further example, in the FIG. 3(a), the relative phase of eachpattern has been shown. Assuming that the radiated wave has a rightcircular polarization, the horizontal component is in advance by (signwith respect to the vertical component and the relative phases of thepatterns SI-Id and DHd resulting from the main horizontal components areas shown in FIG. 3( b). By examination of the components E of the FIG.1, the phases of the spurious patterns Dvd and Svd may be found invertical polarization. The combination of the patterns DV and Dvd of thedifference channel in vertical polarization shows that amplitudes areadded for positive bearings and subtracted for negative bearings in sucha manner that the null axis is shifted toward the negative bearings. Thesignificance of that fact is that the null value of the ration D/S is nolonger obtained for g 0, but for a negative value of g defined by thebearing a of the point A. In a same manner, the combination of thepatterns DHd and DH in horizontal polarization shows that the null axisis also shifted toward the negative bearings by the same angle a.

The combination of the patterns in the sum channel, SVand Svd, forexample, on the one hand, and SIM and Sh on the other hand, shows thatthe resulting patterns are dissymmetric, but such a dissymmetry has verylittle affect on the ratio D/S for low bearings values.

for negative bearings in such a manner that the minimum (null) axis isshifted toward the positive bearings, that is to say in the reversedirection when compared with the case of right circular polarization.

As has been previously mentioned, the angular value of the shift isgiven by the bearing a of the point A. It

can also be determined by calculation, which results in the followingapproximate formula:

a=-kd/'rr L E kd/2 'n'jb,

where d is the system wavelength, f the focal distance of the reflector,2b the angle deviation between maxima of the two primary lobes of thedifference channel and k is the ratio of amplitudes of main and spuriousvertical components. Quantities a and b will be expressed in radians.The above relationship has been established by assuming that the oddsymmetry radiation pattern associated with the main components areprovided by two punctual (point) sources S and 8+ (FIG. 4) opposite inphase and spaced by a distanceL. Those patterns are of the type sin(sin) the corresponding spurious patterns are of even symmetry and arerepresented by The value of the angle a is then determined from theequation:

sin sin a)+lc cos sin a)=0.

The objective of the present invention being the elimination of theantenna axis shift upon polarization mode change, the present inventionuniquely provides for the insertion of a removable dielectric phasecorrection lens over the polarization grid, between the polarizer andthe reflector, the said dielectric lens being integral with thepolarizer. FIG. 4 shows a crosssection, in a horizontal plane, of anassembly of reflector 1, horn 2, polarizer 3 and dielectric lens 4.

Since the electric field pattern for the horn aperture in the differencechannel is that illustrated by the curve D of FIG. 2, it may beconsidered, for the sake of simplicity, that the obtained primaryradiation pattern is equivalent to that which would be obtained from twopunctual sources sand 5+ opposite in phase and each located at equaldistance from the horn center and from the nearest side-wall. For thesecondarypattern, the corresponding punctual sources are referenced S-and S+, spaced by a distance L. These points S' and S+ result from ageometric construction taking into account that the angle between themaxima of the two primary lobes is equal to 2b.

It has been previously shown that changing from linear polarization tocirculate polarization resulted in shifting the antenna axis by an anglea toward the negative bearings, which means, according to antennatheory, that the phase difference between the sources S+ and S'- is .nolonger equal to 1r. It may be shown that the far electric field of anantenna comprising the punctual sources S'- and S+ is of the type:

Z+ej (0+? sin 9):

j being the complex term such that j" -l 0 being the phase shift betweenthe source S+ and the source S 'i-,

and g being the bearing which can be positive or negative.

If the polarizer is removed, the antenna axis corresponds to g 0, thephase shift between S+ and, S- being equal to 11-. With the polarizer inplace, the axis is shifted by an angle a toward the negative bearings,which means that in the last above formula, c (Z-rrL/d sin a 11-, (signif the negative value of a is taken into account). Stated otherwise, thephase shift c is greater than 11. To correct the antenna axis, it isnecessary that the phase shift be equal to 1r, a condition obtained byintroducing a phase shift (21rL/d) sin a from the source S+ to thesource S, i.e., an additional phase shift (2rrL/d) sin a from the sourceS to the source 8+ or in an equivalent manner from the source .rto thesource 5+ (at the horn aperture).

Indeed the sources 5+ and sas well as S+ and S v (41rL/d) sin a=(161rf/d) sin a tan 11/2 5 (81rfb/d) sin I at the lateral extremity 6of the said polarizer. Thus the required phase shift between thesourcess and s+ is obtained. As shown by the FIG. 4, a prism 4 of dielectricmaterial with a dielectric constant B which has a base It such that(41rL/d) sin a (Z'n'h/d) V I may be used, or it 2L/ \/F l) a, for lowvalues of the angle a.

In order that the prism does not introduce significant discontinuities,the dielectric constant B must be selected in the vicinity of unity.Good results have been obtained with a prism made of polyurethane foamhaving a dielectric constant B= 1.10.

In the case of left circular polarization [FIG. 3(c)],

adjacent the end 5.

While the present invention has been described in connection with aspecific apparatus, it is to be clearly understood that it is notlimited to the said example,

The drawings and this description are to be regarded as a A typical andillustrative only.

7 What is claimed is: 1. In a radar antenna system which includes agenerally parabolic reflector illuminated by an offset primary source toprovide a plane wave, the combination comprising:

a polarizing device located between said primary source and saidreflector;

phase correction means inserted between said polarizer and saidreflector for introducing a phase shift so as to produce a beam axis forsaid antenna system which is coincident with said beam axis in theabsence of said polarizer.

2. A monopulse radar antenna system which includes anasymmetrically-shaped parabolic reflector and an offset monopulse feedhorn for radiating and receiving a monopulse beam pattern, comprisingthe combination of:

means comprising a polarizer over the aperture of said horn forcircularly polarizing energy radiated from said horn and for responding.to correspondingly polarized received energy; phase correction meanscomprising a dielectric lens employed over said polarizer in the fieldbetween said polarizer and said reflector, for introducing a phase shiftto cause the axis of said monopulse beam pattern in circularpolarization operation to coincide with the axis of the monopulse beampattern produced in the absence of both said phase correction means andsaid polarizer. 3. Apparatus according to claim 2 in which saidreflector extends in a first dimension symmetrically and in a seconddimension unilaterally with respect to the axis of said horn, and saidhorn is oriented to illuminate substantially the entire surface of saidreflector.

4. Apparatus according to claim 3 in which said horn is of rectangularcross-section with a median partition, thereby to provide two feedchannels for monopulse implementation, and in which said lens comprisesa tapered dielectric prism, said taper extending across the dimension ofsaid horn normal to said median partition.

5. Apparatus according to claim 4 in which said taper produces asubstantially linear prism thickness in the path of energy passingthrough said horn, said thickness varying from substantially zeroadjacent one short side wall of said rectangular horn to a maximumadjacent the opposite short side wall of said horn.

6. Apparatus according to claim 5 in which said prism is constructed ofdielectric material of approximately unity dielectric constant.

7. Apparatus according to claim 5 in which said prism is constructed ofpolyurethane foam having a dielectric constant approximately 1.10.

8. Apparatus according to claim 5 inwhich said prism thickness maximumis substantially equal to 2La/ B l, where L is the deviation on saidreflector between the maxima of the two primary lobes of the differencechannel of said monopulse radar antenna, a is the angular deviationbetween the antenna axis in linear and circular polarization.

1. In a radar antenna system which includes a generally parabolicreflector illuminated by an offset primary source to provide a planewave, the combination comprising: a polarizing device located betweensaid primary source and said reflector; phase correction means insertedbetween said polarizer and said reflector for introducing a phase shiftso as to produce a beam axis for said antenna system which is coincidentwith said beam axis in the absence of said polarizer.
 2. A monopulseradar antenna system which includes an asymmetrically-shaped parabolicreflector and an offset monopulse feed horn for radiating and receivinga monopulse beam pattern, comprising the combination of: meanscomprising a polarizer over the aperture of said horn for circularlypolarizing energy radiated from said horn and for responding tocorrespondingly polarized received energy; phase correction meanscomprising a dielectric lens employed over said polarizer in the fieldbetween said polarizer and said reflector, for introducing a phase shiftto cause the axis of said monopulse beam pattern in circularpolarization operation to coincide with the axis of the monopulse beampattern produced in the absence of both said phase correction means andsaid polarizer.
 3. Apparatus according to claim 2 in which saidreflector extends in a first dimension symmetrically and in a seconddimension unilaterally with respect to the axis of said horn, and saidhorn is oriented to illuminate substantially the entire surface of saidreflector.
 4. Apparatus according to claim 3 in which said horn is ofrectangular cross-section with a median partition, thereby to providetwo feed channels for monopulse implementation, and in which said lenscomprises a tapered dielectric prism, said taper extending across thedimension of said horn normal to said median partition.
 5. Apparatusaccording to claim 4 in which said taper produces a substantially linearprism thickness in the path of energy passing through said horn, saidthickness varying from substantially zero adjacent one short side wallof said rectangular horn to a maximum adjacent the opposite short sidewall of said horn.
 6. Apparatus according to claim 5 in which said prismis constructed of dielectric material of approximately unity dielectricconstant.
 7. Apparatus according to claim 5 in which said prism isconstructed of polyurethane foam having a dielectric constantapproximately 1.10.
 8. Apparatus according to claim 5 in which saidprism thickness maximum is substantially equal to 2La/ Square Root B -1, where L is the deviation on said reflector between the maxima of thetwo primary lobes of the difference channel of said monopulse radarantenna, a is the angular deviation between the aNtenna axis in linearand circular polarization.